In this capacitor assembly the foil sheets and the dielectric film are wound about a central longitudinal axis to form a roll in which the sheets and film are disposed in interleaving turns extending about the axis. Each of the foil sheets has two side edges disposed at opposite ends of the roll, a head edge near the central axis of the roll, and a tail edge near the outer periphery of the roll.
A widely-used design for this type of capacitor roll is the extended-foil design. In this design, one of the foils has one of its two side edge regions extending out beyond the dielectric film at one end of the capacitor roll, and the other of the foils has one of its two side edge regions extending out beyond the dielectric film at the other end of the capacitor roll. The other side edge region of each foil is located in a position spaced inwardly from the adjacent outer edge of the dielectric film, or, in other words, in buried relationship between the turns of the dielectric film.
For reducing electric-field stress concentrations at the edges of the sheets, it is conventional to fold over all edges of the sheets that are disposed in buried relationship between the turns of the dielectric film. In an extended-foil type of capacitor, this has involved folding over one side edge of each foil sheet and the head and the tail edges of each foil sheet. The apparatus for folding over these edges is relatively expensive. It typically comprises a folding device for each side edge of the foil sheet that is to be folded and another folding device for the head and tail edges of each foil sheet.